"Increasing the incline on a treadmill or running or walking uphill is a great way to burn calories," says Kelly Gonzalez, a fitness consultant in Manhattan Beach, Calif. Plus, by heading uphill, you'll target your glutes, hamstrings and calves without putting too much stress on your joints.

To give it a try, hop on a treadmill and warm up for three minutes at an easy pace and 3 percent incline. Every three minutes, increase the incline by 3 percent until you're at 15 percent. Then, work your way back to the start. Do this four times a week and say hello to short shorts.